The invention is directed mainly to gap type electroplating as opposed to tank or bath plating wherein a remotely located anode, either consumable or non-consumable, is placed in a tank with a charged workpiece. Metal is plated onto all surfaces of the workpiece which are in the tank, in accordance with electrolysis technology. To plate only a selected surface in such a tank system, the workpiece must be masked, coated or otherwise shielded from the solution in the tank. Gap type electroplating involves a completely different concept. An anode is provided with a shape and surface generally matching the shape and selected surface of the workpiece being plated. Current flow between the anode and cathode is through a predetermined gap established by the geometry of the anode surface as it relates to the workpiece surface being plated. This type of plating, i.e. gap plating, can be accomplished in a tank and is often done in a plating tank; however, gap plating need not use a tank. It can be performed by directing a plating solution into the gap between the anode and cathode as a current is applied between these two electrodes as long as a closed fluid flow can be made through the gap. This type of gap plating is the main subject of the present invention.
Two examples of the closed circuit gap type plating, to which the present invention is directed, are shown in LaBoda U.S. Pat. No. 4,111,761 and Iemmi U.S. Pat. No. 4,441,976. A somewhat related tank type electroplating process is illustrated in Blanc U.S. Pat. No. 4,345,977. These three patents are incorporated by reference herein as background information since they do contain certain technical descriptions and structures which illustrate the background of the present invention.
As mentioned before, the present invention relates mainly to the art of closed circuit, gap type electroplating as shown generally in LaBoda U.S. Pat. No. 4,111,761 and Iemmi U.S. Pat. No. 4,441,976 wherein an anode having an outer cylindrical surface is fixed concentrically within a cylindrical surface of a workpiece to be plated to define a gap or plating cell. The rest of the workpiece including the complete outer surface is not to be plated. To prevent plating of the remainder of the workpiece, the electroplating solution is not circulated in contact with the area of the workpiece which is not to be plated. In Blanc U.S. Pat. No. 4,345,977, a modified tank system is used. Plating of the outer portion of the workpiece is prevented by seals. The inner cylindrical surface is primarily plated by this apparatus due to anode placement and solution flow; but, other portions of the workpiece are also plated because the tank actually encompasses more than the selected internal surface. This patent is not a gap plating disclosure, but it does show a generally relevant apparatus to plate a selected surface.
The concept of gap plating has been known for many years; however, the fixtures for such processes have been relatively expensive and the results have not been uniform especially in elongated generally inaccessible bores in complex workpieces. For that reason, repair and build up of oversized bores in various workpieces has often been accomplished either by tank plating or brush plating. Tank type plating is extremely slow and does not produce uniform results on only selective surfaces without extensive, expensive masking. Brush type plating depends upon the skill of the operator and can be used for only specific, exposed surfaces. Consequently, there is a substantial demand for a plating system which can plate uniformly, to substantial thicknesses, in excess of 0.050 inches, on various bores of a complex workpiece, such as an aircraft landing gear forging, which system can be done rapidly with low equipment cost by personnel with ordinary skills.
It has become quite desirable to plate in somewhat inaccessible locations of a large workpiece to create an excellent wear resistant, lubricant surface of substantial thickness to reclaim complex workpieces, such as forgings, having only selected surfaces that are worn beyond acceptable tolerances. To satisfy these requirements, chromium cannot always be used because microcracks would be created at the thicknesses which are required to bring an oversized bore into acceptable tolerances. Thus, even though most salvage or repair of selected work surfaces in complex workpieces is done by chromium, chromium is not always an optimum material; therefore, tank plating of such surfaces with chromium is not universally applicable. This is especially true of repairing oversized bores in ultra high strength steel (240 KSI or greater) forgings used in aerospace and aircraft components. In view of these limitations and demands, chromium from tank plating is not completely satisfactory for repairing workpieces, i.e. plating the inner surface of a bore on an ultra high strength steel forging. Chromium plating to repair worn surfaces, even if possible and/or desirable, requires extremely long plating times. Increased current densities to decrease this plating time do not substantially increase the rate at which chromium is deposited because efficiency drops rapidly with increased current density.
In summary, even though tank plating of chromium onto surfaces of a complex workpiece has been used to repair, salvage or re-size surfaces, such process is not completely satisfactory. Indeed, it cannot be used effectively in some situations. Tank plating of nickel is also difficult and costly as a repair, salvage or sizing procedure.